The present invention relates to a fuel cell, a fuel-cell separator for separating the fuel gas and oxidizer gas (air or oxygen) in a fuel cell and a method of producing the fuel-cell separator.
Recently, the considerably increasing consumption of fossil fuels for automobile or the like has caused various problematic environmental disruptions due to the large amounts of various waste gases generating on combustion of the fuels. As a means for solving the problem, fuel cells, being a safe and pollution-free energy generating system, have become of major interest and been actively studied and developed on the worldwide level, and some are put to practical use.
Because of their high energy efficiency, fuel cells can reduce environmental pollution and are expected to be widely used as small dynamos or the power supply for EVs. It is the principle of fuel cells to develop a potential difference by the conversion of chemical energy to electric energy through the oxidation and reduction of a fuel gas and an oxidizer gas (air or oxygen) which are allowed to flow separately over electrodes (positive electrode and negative electrode) attached on the upper and lower sides of an electrolyte layer, and the transfers of cations and electrons in the electrolyte layer. Because fuel cells are produced by piling up electrodes and electrolyte layers alternately in multi-layers, separating plates (separators) for separating the fuel gas from the oxidizer gas are interposed between positive electrodes and negative electrodes which are placed one on the other. To secure gas feed paths, separators generally have ribs (projecting parts; adjoining ribs form a groove therebetween which works as a path for a gas, such as hydrogen or oxygen, or product water). Charge collector plates surrounding the multi-layered electric cells collect the potential difference developed in each electric cell.
Among the members constructing a fuel cell that are important and occupy a majority are the separators, which perform important tasks influencing the characteristics of fuel cells, for example, stable supply of gases (oxygen, hydrogen or the like) to catalysts and electrolyte layers and immediate discharge of the product water out of the system. Separators, therefore, require various properties, including 1) separation of fuel gases from oxidizer gases (gas non-permeability), 2) electric conductivity and 3) resistance to swelling with water produced on negative electrodes or with electrolytic solutions.
Separators have generally been produced by mechanically grooving a graphite block or glassy carbon to form ribs, thereby providing feed paths for fuel gases and oxidizer gases. An alternative is high pressure molding of an expanded-graphite or an expanded-graphite sheet produced by treating a flaky natural graphite with acid and then with heat, or by impregnating the molded expanded-graphite with a liquid thermosetting resin and curing to prevent swelling with liquids (Japanese Patent Application Non-examined Publication Nos. 60-65781 and 60-12672).
Disclosed in the specification of International Publication No. WO97/02612 is a method wherein an expanded-graphite powder of specific particle diameters is dispersed in a thermoplastic or thermosetting resin, molded into a block and then mechanically grooved.
The methods using various machining techniques are costly because they need highly accurate cutting machines or techniques, a very long machining time or a tremendous labor, for example, impregnating the cutting-processed separators with resins by using a vacuum drier. Further, the separators cut out of graphite plates are thick, and have the defect that each separator is so heavy as to problematically increase the weight per fuel cell (generally containing several hundreds of separators). This causes energy loss when fuel cells are fabricated in cars or the like. Additional drawbacks of the separators cut out of graphite plates are hardness and fragility. When several hundreds of separators are stacked and clamped to prevent a gas leak, some are often broken under the clamping pressure. The methods using the expanded-graphite involve the problems that moldable ribs are limited in dimension, and the products apt to swell with the gas generated during molding and cannot be supplied stably.
The separator disclosed in the specification of International Publication No. WO97/02612 has the defect that because the particles of the expanded-graphite powder used for production have small diameters and are very fragile and weak, the expanded-graphite powder is crashed during mixing with resins and gives molded articles of poor strength.
Accordingly, an object of the invention is to provide a fuel-cell separator which is free from problems relating to the properties of fuel-cell separators, such as electric resistance, gas permeability, swelling with liquids and mechanical strength, and is very moldable and economical.
Another object of the invention is to provide a ribbed fuel-cell separator which is further improved in that it can be made lighter because its plate part can be thinned even for high ribs.
Another object of the invention is to provide a fuel-cell separator which is further improved in dimensional accuracy.
Another object of the invention is to provide a fuel-cell separator which is further improved particularly in electrical properties and mechanical strength.
Another object of the invention is to provide a method for economically and stably producing through simple steps a fuel-cell separator which is free from problems relating to the properties of fuel-cell separators such as electric resistance, gas permeability, swelling with liquids and mechanical strength, and has good moldability.
Another object of the invention is to provide a method for producing a fuel-cell separator by using a resin which cures readily without troubles such as corrosion of molds.
Another object of the invention is to provide a further improved method for producing a fuel-cell separator which is particularly excellent in electric properties and mechanical strength.
Another object of the invention is to provide a fuel cell of high quality which contains fuel-cell separators excelling in the properties of fuel-cell separators relating to electric resistance, gas permeability, swelling with liquids and mechanical strength.
Another object of the invention is to provide a fuel cell which is further improved in stably maintaining its cell-properties during a long-term usage.
Accordingly, the invention relates to the following subjects.
(1) A fuel-cell separator comprising a resin and an electric conductor dispersed in the resin.
(2) The fuel-cell separator as described in (1), wherein the electric conductor is a powdery electric conductor having an average particle diameter of 25 xcexcm or more.
(3) The fuel-cell separator as described in (1) or (2), wherein the electric conductor is an expanded-graphite powder.
(4) The fuel-cell separator as described in (3), wherein the expanded-graphite powder has a sulfuric acid ion (SO42xe2x88x92) concentration of 500 ppm or less.
(5) The fuel-cell separator as described in any one of (1) to (3), wherein the resin is a cured phenolic resin.
(6) The fuel-cell separator as described in (5), wherein the resin is a cured phenolic resin cured by ring-opening-polymerization.
(7) The fuel-cell separator as described in any one of (1) to (6), which has a shape of a ribbed-plate formed by monobloc-molding a plate and ribs.
(8) The fuel-cell separator as described in (7), wherein the ribs have a height of 0.3 mm or more.
(9) The fuel-cell separator as described in (7), wherein the ribs have a height of 0.6 mm or more.
(10) The fuel-cell separator as described in any one of (7) to (9), wherein the ratio of the height (A) of the ribs to the thickness (B) of the plate, (A/B), is 0.5 or more.
(11) The fuel-cell separator as described in any one of (7) to (10), which has the ribs on one side of the plate.
(12) The fuel-cell separator as described in any one of (7) to (11), which has the ribs on both sides of the plate.
(13) The fuel-cell separator as described in any one of (7) to (12), wherein the plate has a thickness of 0.25 mm to 2.0 mm.
(14) The fuel-cell separator as described in any one of (7) to (13), wherein the ribs are tapered at an angle of 2xc2x0 to 30xc2x0.
(15) The fuel-cell separator as described in (14), wherein the ribs are tapered at an angle of 2xc2x0 to 20xc2x0.
(16) The fuel-cell separator as described in any one of (1) to (15), which has a bending strength of 30 MPa or more.
(17) The fuel-cell separator as described in (16), wherein the electric conductor comprises a carbon fiber and an expanded-graphite powder.
(18) The fuel-cell separator as described in any one of (1) to (16), wherein the electric conductor is a powdery electric conductor having a flaky branched-needle-like shape or a dendritic shape.
(19) The fuel-cell separator as described in any one of (1) to (18), wherein the dispersed electric conductor is oriented partially in a direction of the thickness of the fuel-cell separator and partially in a direction perpendicular to the direction of the thickness.
(20) The fuel-cell separator as described in any one of (1) to (19) that has a surface in and near which the electric conductor dispersed in the resin is oriented along the surface.
(21) The fuel-cell separator as described in (19) or (20), wherein the oriented electric conductor lies in fibrous rows.
(22) The fuel-cell separator as described in any one of (1) to (21), wherein the dispersed electric conductor partially lies in tangled fibrous rows.
(23) The fuel-cell separator as described in any one of (1) to (22), which is to be used in a solid-polymer fuel cell.
(24) The fuel-cell separator as described in any one of (1) to (23), which has a residual carbolic acid concentration of 100 ppm or less.
(25) The fuel-cell separator as described in any one of (1) to (24), which has a residual sulfuric acid ion (SO42xe2x88x92) concentration of 200 ppm or less.
(26) A phosphoric acid-fuel-cell separator produced from the fuel-cell separator as described in any one of (1) to (25) by carbonizing the resin contained in the fuel-cell separator.
(27) A fuel-cell separator, which has a bending strength of 30 MPa or more.
(28) The fuel-cell separator as described in (27), which has a shape of a ribbed-plate formed by monobloc-molding a plate and ribs.
(29) The fuel-cell separator as described in (28), wherein the ribs have a height of 0.3 mm or more.
(30) The fuel-cell separator as described in (29), wherein the ribs have a height of 0.6 mm or more.
(31) The fuel-cell separator as described in any one of (28) to (30), wherein the ratio of the height (A) of the ribs to the thickness (B) of the plate, (A/B), is 0.5 or more.
(32) The fuel-cell separator as described in any one of (28) to (31) that has the ribs on one side of the plate.
(33) The fuel-cell separator as described in any one of (28) to (31) that has the ribs on both sides of the plate.
(34) The fuel-cell separator as described in any one of (28) to (33), wherein the plate has a thickness of 0.25 mm to 2.0 mm.
(35) The fuel-cell separator as described in any one of (28) to (34), wherein the ribs are tapered at an angle of 2xc2x0 to 30xc2x0.
(36) The fuel-cell separator as described in (35), wherein the ribs are tapered at an angle of 2xc2x0 to 20xc2x0.
(37) The fuel-cell separator as described in any one of (27) to (36), which comprises a fibrous material, an expanded-graphite powder and a resin, wherein the fibrous material and the expanded-graphite powder are dispersed in the resin.
(38) The fuel-cell separator as described in (37), wherein the expanded-graphite powder has an average particle diameter of 25 xcexcm or more.
(39) The fuel-cell separator as described in (37) or (38), wherein the expanded-graphite powder has a sulfuric acid ion (SO42xe2x88x92) concentration of 500 ppm or less.
(40) The fuel-cell separator as described in any one of (37) to (39), wherein the resin is a cured phenolic resin.
(41) The fuel-cell separator as described in (40), wherein the resin is a cured phenolic resin cured by ring-opening-polymerization.
(42) The fuel-cell separator as described in any one of (37) to (41), wherein the expanded-graphite powder has a flaky branched-needle-like shape or a dendritic shape.
(43) The fuel-cell separator as described in any one of (37) to (42), wherein the dispersed, expanded-graphite powder is oriented partially in a direction of the thickness of the fuel-cell separator and partially in a direction perpendicular to the direction of the thickness.
(44) The fuel-cell separator as described in any one of (37) to (44) that has a surface near which the dispersed, expanded-graphite powder is oriented along the surface.
(45) The fuel-cell separator as described in (43) or (44), wherein the expanded-graphite powder is oriented in fibrous rows.
(46) The fuel-cell separator as described in any one of (37) to (45), wherein the dispersed expanded-graphite powder partially lies in tangled fibrous rows.
(47) The fuel-cell separator as described in any one of (27) to (46), which is to be used in a solid-polymer fuel cell.
(48) The fuel-cell separator as described in any one of (27) to (47), which has a residual carbolic acid concentration of 100 ppm or less.
(49) The fuel-cell separator as described in any one of (27) to (48), which has a residual sulfuric acid ion (SO42xe2x88x92) concentration of 200 ppm or less.
(50) A phosphoric acid-fuel-cell separator produced from the fuel-cell separator as described in any one of (27) to (49) by carbonizing the resin.
(51) A method of producing the fuel-cell separator as described in (1), comprising thermally molding a mixture comprising an electric conductor and a resin.
(52) The method of (51) for producing the fuel-cell separator, wherein the electric conductor is an expanded-graphite powder.
(53) The method of (51) for producing the fuel-cell separator, wherein the electric conductor comprises a carbon fiber and an expanded-graphite powder.
(54) The method of (52) or (53) for producing the fuel-cell separator, wherein the expanded-graphite powder has a bulk density of 0.1 to 1.0 g/cm3.
(55) The method of (52), (53) or (54) for producing the fuel-cell separator, wherein the expanded-graphite powder has an average particle diameter of 25 xcexcm or more.
(56) The method of any one of (52) to (55) for producing the fuel-cell separator, wherein the expanded-graphite powder is obtained by pulverizing a molded, expanded-graphite.
(57) The method of (56) for producing the fuel-cell separator, wherein the molded, expanded-graphite has a density of 0.6 to 2.0 g/cm3.
(58) The method of any one of (51) to (57) for producing the fuel-cell separator, wherein the resin has a softening point of 300xc2x0 C. or lower.
(59) The method of any one of (52) to (58) for producing the fuel-cell separator, wherein the expanded-graphite powder has a sulfuric acid ion (SO42xe2x88x92) concentration of 500 ppm or less.
(60) The method of (59) for producing the fuel-cell separator, wherein the expanded-graphite powder is obtained by pulverizing a molded, expanded-graphite molded article, by washing it with water and drying.
(61) The method of (59) for producing the fuel-cell separator, wherein the expanded-graphite powder is obtained by heat-treating a molded, expanded-graphite at a temperature of 350xc2x0 C. or higher and then pulverizing it after cooling.
(62) The method of (59) for producing the fuel-cell separator, wherein the expanded-graphite powder is obtained by pulverizing a molded, expanded-graphite and then heat-treating it at a temperature of 350xc2x0 C. or higher.
(63) The method of any one of (51) to (62) for producing the fuel-cell separator, wherein the fuel-cell separator has a shape of a ribbed-plate, and wherein the thermal molding is accomplished by monobloc-molding ribs and a plate with heat and pressure.
(64) The method of any one of (51) to (63) for producing the fuel-cell separator, comprising
a pre-molding step wherein the mixture comprising the electric conductor and the resin is compressed at a temperature at which the resin does not melt nor cure; and
a thermal molding step wherein a pre-molded article produced in the pre-molding step is compressed at a temperature at which the resin melts or cures.
(65) The method of (64) for producing the fuel-cell separator, wherein the pre-molding is carried out at a temperature not lower than 0xc2x0 C. but lower than 80xc2x0 C.
(66) The method of any one of (51) to (63) for producing the fuel-cell separator, wherein the thermal molding of the mixture comprising the electric conductor and the resin is accomplished by molding the mixture comprising the electric conductor and the resin into a tablet, and full-molding the tablet at a higher temperature under a higher pressure than the temperature and the pressure of the tablet molding.
(67) The method of (66) for producing the fuel-cell separator, wherein the molding for producing the tablet is carried out at a temperature at which the resin partially melts or reacts with heat.
(68) The method of any one of (51) to (67) for producing the fuel-cell separator, wherein the fuel-cell separator is to be used in a solid-polymer fuel cell.
(69) The method of any one of (51) to (68) for producing the fuel-cell separator, wherein a molded article obtained by the thermal molding is further heated at a temperature of 200xc2x0 C. or higher.
(70) A method of producing the fuel-cell separator of (50), wherein the fuel-cell separator is to be used in a phosphoric acid-type fuel cell, and which comprises thermally molding a mixture comprising the electric conductor and the resin, and then carbonizing the resin.
(71) A fuel cell having the fuel-cell separator of any one of (1) to (25).
(72) The fuel cell as described in (71), which is a solid-polymer fuel cell.
(73) A fuel cell having the fuel-cell separator of (26).
(74) The fuel-cell as described in (73), which is a solid-polymer fuel cell.
(75) A fuel cell having the fuel-cell separator of any one of (27) to (49).
(76) The fuel cell as described in (75), which is a solid-polymer fuel cell.
(77) A fuel cell having the fuel-cell separator produced by the method of (50).
(78) The fuel cell as described in (77), which is a solid-polymer fuel cell.
(79) A fuel cell having the fuel-cell separator produced by the method of any one of (51) to (69).
(80) The fuel cell as described in (79), which is a solid-polymer fuel cell.
(81) A fuel cell having the phosphoric acid-type fuel-cell separator produced by the method of (70).
(82) The fuel cell as described in (81), which is a solid-polymer fuel cell.